In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been, and continues to be, efforts toward scaling down the device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution lithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and the film exposed with a radiation source (such as optical light, x-rays, or an electron beam) that illuminates selected areas of the surface through an intervening master template, the mask, forming a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. A positive-tone resist is one that becomes more soluble in the developer after exposure to actinic radiation. A negative-tone resist becomes less soluble in the developer after exposure. The more soluble areas are removed in the developing process to leave the pattern image in the coating.
A resist coating is typically prepared by dripping or spraying a resist solution onto a spinning substrate. This forms a relatively uniform coating of the resist solution, which is then “soft-baked.” Soft-baking drives off solvent, improves adhesion of the resist to the substrate, and anneals stresses caused by shear forces encountered in the spinning process. Typically, the solvent level is reduced from the 20% to 30% range to about the 4% to 7% range.
The time and temperature of the soft-bake determines a number of parameters that affect subsequent processing steps. The degree of soft-baking affects the residual solvent content of the resist, which in turn affects the rate of attack of the resist by the developer. Under-baked resists may show inadequate differentiation between the dissolution rates of exposed and un-exposed regions. On the other hand, over-baking reduces photosensitivity of the resist, which also reduces the ability to create sharp contrast between exposed and unexposed regions. Consequently, the soft-bake must be carefully optimized and controlled.
Particularly where extremely fine patterns are sought, the pre-bake process must not only be controlled from substrate to substrate, but also across each individual substrate. Both the overall temperature history and variations in the temperature across the photoresist must be controlled. Variation in the temperature history across the substrate during pre-bake can lead, after exposure of the resist, to unintended lengthwise variations in the width of features such as lines and gaps. Chemically amplified photoresists are particularly susceptible to such variations. The feature sizes of chemically amplified photoresists can be drastically affected by only a few degrees difference in temperature. Line size deviations often occur unless temperature is maintained within 0.5° C. tolerance across the substrate. Temperature control within ±0.2° C. may be required.
Much attention has been given to systems for uniformly heating photoresist coated substrates. While convection ovens have been used, they have limitations. The temperature uniformity of convection ovens is not particularly good and particles may enter the ovens and become embedded in the heated resist. Infrared ovens have been widely utilized. These ovens have much shorter heating times than convection ovens (3-4 minutes versus approximately 30 minutes). Hot-plates also permit rapid heating.
Less attention has been given to cooling systems, although several have been suggested. Natural convection cooling under ambient conditions has been used, but this is relatively slow and results in substantial non-uniformities. Cold-plates are somewhat better. These can be cooled by cooling fluids or Peltier elements. However, substrate temperature gradients form when using cold plates, since heat must travel from the substrates and the surroundings to the cold plates. It has been proposed to submerge the substrates in a liquid such as water. Cooling in this case may be too rapid and cause mechanical damage to the substrate. Submerging also has the disadvantage of requiring a drying step. Use of a cooling gas has been suggested, but a cooling gas does not appear to have been successfully used to achieve uniform cooling.
Therefore, there remains an unsatisfied need for a system and method of rapidly and uniformly cooling resist coated substrates.